Coprecipitation reaction examples

Mucci and Morse (1989) have reviewed much of the research on coprecipitation reactions with calcite and aragonite, and the interested reader is referred to their paper for a detailed discussion of this literature. Here we present examples of the complex coprecipitation behavior of some of the most important ions in natural systems with carbonate minerals. The ions that we have selected are Mg2+, Sr2+, Na+, Mn2+, and SO42-. 

As discussed previously in this chapter, there are spots where the lead content (and the elementary composition) is higher and so-called lead enrichments are formed. Table 2.15 shows that the composition of lead enrichments can be rather different in different enrichments. Besides the increase in lead concentration, other changes in elementary composition may also occur. There are examples of lead enrichment (Figure 2.27 Sample 3 in Table 4) where the average composition remains unchanged even though the lead concentration is elevated. However, there are lead enrichments where the increase in lead concentration is accompanied by a simultaneous increase in iron (Sample 4 in Table 2.15), or a simultaneous increase of calcium and lead (Sample 2). Lead enrichment in these cases is likely due to lead coprecipitation reactions with other minerals (iron oxide and other calcium silicate phases) that were present in the sample. 

Another, and on the face of it, rather different example, is the coprecipitation of solid solution compounds, such as CulnSi and CulnSei—semiconductors of particular interest due mainly to their applicability for photovoltaic cells. It was shown, by X-ray diffraction, that the precipitate resulting from reaction between H2S and an aqueous solution containing both Cu" and In " ions was, at least in part (depending on the concentrations of the cations), single-phase CulnSi [3]. Two factors were found to be necessary for this compound formation (1) the presence of sulphide on the surface of the initially precipitated colloidal solid metal sulphide and (2) one of the cations being acidic and the other basic. The monovalent Cu cation is relatively basic, while the trivalent In cation is relatively acidic. It is not clear what the physical reason is for this latter requirement. A difference in practice between acidic and basic cations is that, in an aqueous solution of both cations, the acidic cation is more likely to be in the form of some hydroxy species (not to be confused with hydrated cations), while the basic cation is more likely to exist as the free cation. 

Chemisorption raises basic questions for the carbonate geochemist about the boundary between sorption and coprecipitation. If the adsorption reaction takes place in a solution that is also supersaturated with respect to the carbonate mineral substrate, then the adsorbed ions can be buried in the growing layers of the mineral and become coprecipitates. This mechanism can result in distribution coefficients that are dependent on growth rates. Also, when chemisorption is involved, an entirely new phase or a coprecipitate can form in the near-surface region of the carbonate (e.g., see Morse, 1986 Davis et al 1987). A classic example is apatite formation on calcite in dilute solutions (e.g., Stumm and Leckie, 1970). 

An important practical case during SEC is coprecipitation of structurally similar compounds, for example, reaction by-products, chiral molecules, additives, and impurities. Such precipitation is important when the aim is to produce composite crystals or drug xcipient mixtures or, on the contrary, to separate impurities from a solid product. The similarity of molecular structures ensures that such compounds strongly interact with each other, thus increasing the likelihood for solid solutions to form. Strong solid-solid intermolecular interactions typically result in separation problems and in significant variations of the solid state and particulate properties. 

X-ray diffraction and microscopic studies revealed that calcined stoichiometric mixtures of coprecipitated hydroxides do indeed form spinels and solid solutions. With some mixtures, complete reaction was not always easily attained. For example, in the CuO Fe203 system, excess copper oxide and Fe203 peaks were found in x-ray diffraction patterns in addition to the major spinel phase. Calcined mixtures usually produced pure spinel compounds. 

Coprecipitation has been used for the production of ferrite particles. For example, according to Tang et al., MnFe04 particles of relatively small size (5-25 nm) can be obtained through the reactions... 

A reproducible coprecipitation or adsorption reaction of a constant amount of the element of interest can be used in substoichiometry. An interesting example is the substoichiometric radioactivation analysis for oxygen, based on the reproducible isolation of fluorine or fluorosilicate with a substoichiometric amount of hydrated tin dioxide It has been applied to the determination of oxygen in silicon crystal. 

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